The present invention pertains to improvements in centrifugal jigs. Specific examples of centrifugal jigs are disclosed in U.S. Pat. No. 4,279,741 to Campbell, issued Jul. 21, 1981 and titled "Method and Apparatus for Centrifugally Separating a Heavy Fraction From a Light Weight Fraction Within a Pulp Material" and in U.S. Pat. No. 4,998,986 to Campbell, issued Mar. 12, 1991 and titled "Centrifugal Jig Pulsing System." Both patents are hereby incorporated into this disclosure by reference.
The general advantages and operational features of centrifugal jigs can be readily ascertained from the above-referenced patents. Depending upon the specific application of such a jig, either the heavy fraction or the lightweight fraction separated by its operation might contain the values desired as an end product.
In a centrifugal jig, a rotor is provided to act upon an incoming pulp or slurry. The rotor includes a rotating jig screen and a surrounding rotating hutch. The hutch is maintained full of fluid during operation. Fluid pulses are directed to the interior space of the hutch by a pulsator, such as a rotating supply valve or by pulse blocks which are mounted to the rotor and which spin with it about its central axis. Other forms of internal or external fluid pulsators may be utilized in conjunction with the improvement of the present disclosure.
In a centrifugal jig of the type disclosed in the above-identified patents, a pre-screened incoming pulp or slurry containing heavy and light fractions in a range of particle sizes is introduced directly onto the separating jig screen. The jig screen has openings formed through it of a size sufficient to permit radial outward passage of the particles in the slurry.
The theory of operation of such a jig assumes that the pulsing of the slurry on the perforated screen will first radially stratify the particles according to their specific gravities, and that then the heavier particles will escape through the screen openings as a result of centrifugal force. However, because stratification of the similarly-sized particles into heavy and lightweight fractions typically occurs along the axial length of the separating screen itself, some particles in the lightweight fraction inevitably will become entrapped within the particles of the heavy fraction as the heavy fraction migrates toward the screen surface.
Entrapped lightweight particles are usually discharged along with the heavy particles. Their presence decreases the overall percentage of heavy particles in the resulting recovered fraction. The extent of this problem is a function of the nature of the materials and particle sizes within the incoming slurry and the relative specific gravities of the lightweight and heavy fractions contained within it. In actual practice, the significance of the resulting dilution of recovered material varies substantially from one specific application to another.
The present disclosure utilizes a split screen to address the problem created by the escape of lightweight particles prior to slurry stratification. The incoming slurry is first directed onto a stratification section of the jig screen along which the slurry is radially pulsated. This stratifies the particles according to their respective weights, with the heavier particles being positioned radially outward from those of lesser weight. However, no particles are permitted to escape radially through this section as such layering is accomplished.
After being pre-stratified, the slurry is then directed onto a recovery screen section for separation of its lightweight and heavy fractions as detailed in the above-identified patents. Depending upon the nature of a particular slurry, substantially higher degrees of separation can be achieved by such pre-stratification. The required radial movement of heavier particles that takes place during separation on the jig screen is not accomplished in competition with the stratification of the particles along the same jig screen surface. Lightweight particles are therefore less likely to become entrapped by the outwardly migrating heavy particles.